Bone grafts have become an important and accepted means for treating bone fractures and defects. In the United States alone, approximately half a million bone grafting procedures are performed annually, directed to a diverse array of medical interventions for complications such as fractures involving bone loss, injuries or other conditions necessitating immobilization by fusion (such as for the spine or joints), and other bone defects that may be present due to trauma, infection, or disease. Bone grafting involves the surgical transplantation of pieces of bone within the body, and generally is effectuated through the use of graft material acquired from a human source. This is primarily due to the limited applicability of xenografts, transplants from another species.
Orthopedic autografts or autogenous grafts involve source bone acquired from the same individual that will receive the transplantation. Thus, this type of transplant moves bony material from one location in a body to another location in the same body, and has the advantage of producing minimal immunological complications. It is not always possible or even desirable to use an autograft. The acquisition of bone material from the body of a patient typically requires a separate operation from the implantation procedure. Furthermore, the removal of material, oftentimes involving the use of healthy material from the pelvic area or ribs, has the tendency to result in additional patient discomfort during rehabilitation, particularly at the location of the material removal. Grafts formed from synthetic material have also been developed, but the difficulty in mimicking the properties of bone limits the efficacy of these implants.
As a result of the challenges posed by autografts and synthetic grafts, many orthopedic procedures alternatively involve the use of allografts, which are bone grafts from other human sources (normally cadavers). The bone grafts, for example, are placed in a host bone and serve as the substructure for supporting new bone tissue growth from the host bone. The grafts are sculpted to assume a shape that is appropriate for insertion at the fracture or defect area, and often require fixation to that area as by screws or pins. Due to the availability of allograft source material, and the widespread acceptance of this material in the medical community, the use of allograft tissues is certain to expand in the field of musculoskeletal surgery.
FIGS. 1A, 1B, 1C, and 1D show the relative sizes of the femur 10 (thigh), tibia 11 (lower leg), humerus 12 (upper arm), and radius 13 (lower arm) respectively for an adult. As can be seen when comparing these bones, their geometry varies considerably. The lengths of these bones may have a range, for example, from 47 centimeters (femur), to 26 centimeters (radius). In addition, as shown in FIGS. 1E and 1F, the shape of the cross section of each type of bone varies considerably, as does the shape of any given bone over its length. While the femur 10, as shown in FIG. 1E, has a generally rounded outer shape, the tibia 11 has a generally triangular outer shape as shown in FIG. 1F. The wall thickness also varies in different areas of the cross-section of each bone. For example, femur 10 has a wall thickness X1 that is much smaller than wall thickness X2. Similarly, tibia 11 has a wall thickness X3 that is much smaller than wall thickness X4. Even after clearing the inner canal regions 14 and 15 within the bones, the contours of these canals vary considerably. Thus, machining of the bone to have standardized outer dimensions and/or canal dimensions is necessary in many applications.
Sections of bones with regions having narrow cross-sections, as seen for example with thicknesses X1 and X3, may be rejected for use in certain applications because the wall thickness does not have sufficient strength. Preferably, no region of a bone section has a thickness less than 5 millimeters, although in some applications smaller wall thicknesses may be employed. Thus, in the case that a bone section is found to have a region with a wall thickness less than a minimum acceptable thickness, such a bone section is rejected as being unsuitable for use in a bulk configuration. Often, such a section is ground into bone particulate that is then used in other applications. The minimum thickness standards imposed on the use of bone sections results in the rejection of substantial quantities of bone sections, and thus an inefficient use of the material. Bone sections that do not meet the minimum thickness standards are often found in older individuals.
As a collagen-rich and mineralized tissue, bone is composed of about forty percent organic material (mainly collagen), with the remainder being inorganic material (mainly a near-hydroxyapatite composition resembling 3Ca3(PO4)2.Ca(OH)2). Structurally, the collagen assumes a fibril formation, with hydroxyapatite crystals disposed along the length of the fibril, and the individual fibrils are disposed parallel to each other forming fibers. Depending on the type of bone, the fibrils are either interwoven, or arranged in lamellae that are disposed perpendicular to each other.
There is little doubt that bone tissues have a complex design, and there are substantial variations in the properties of bone tissues with respect to the type of bone (i.e., leg, arm, vertebra) as well as the overall structure of each type. For example, when tested in the longitudinal direction, leg and arm bones have a modulus of elasticity of about 17 to 19 GPa, while vertebra tissue has a modulus of elasticity of less than 1 GPa. The tensile strength of leg and arm bones varies between about 120 MPa and about 150 MPa, while vertebra have a tensile strength of less than 4 MPa. Notably, the compressive strength of bone varies, with the femur and humerus each having a maximum compressive strength of about 167 MPa and 132 MPa respectively. Again, the vertebra have a far lower compressive strength of no more than about 10 MPa.
With respect to the overall structure of a given bone, the mechanical properties vary throughout the bone. For example, a long bone (leg bone) such as the femur has both compact bone and spongy bone. Cortical bone, the compact and dense bone that surrounds the marrow cavity, is generally solid and thus carries the majority of the load in major bones. Cancellous bone, the spongy inner bone, is generally porous and ductile, and when compared to cortical bone is only about one-third to one-quarter as dense, one-tenth to one-twentieth as stiff, but five times as ductile. While cancellous bone has a tensile strength of about 10–20 MPa and a density of about 0.7, cortical bone has a tensile strength of about 100–200 MPa and a density of about 2. Additionally, the strain to failure of cancellous bone is about 5–7%, while cortical bone can only withstand 1–3% strain before failure. It should also be noted that these mechanical characteristics may degrade as a result of numerous factors such as any chemical treatment applied to the bone material, and the manner of storage after removal but prior to implantation (i.e. drying of the bone).
Notably, implants of cancellous bone incorporate more readily with the surrounding host bone, due to the superior osteoconductive nature of cancellous bone as compared to cortical bone. Furthermore, cancellous bone from different regions of the body is known to have a range of porosities. Thus, the design of an implant using cancellous bone may be tailored to specifically incorporate material of a desired porosity.
It is essential to recognize the distinctions in the types and properties of bones when considering the design of implants. Surgeons often work with bones using similar tools as would be found in carpentry, adapted for use in the operating room environment. This suggests that bones have some properties which are similar to some types of wood, for example ease in sawing and drilling. Notably, however, are many differences from wood such as the abrasive nature of hydroxyapatite and the poor response to local heating during machining of a bone. The combination of tensile and compressive strengths found in bone, resulting from the properties of the collagen and hydroxyapatite, is thus more aptly compared to the tensile and compressive strengths found in reinforced concrete, due to steel and cement. Furthermore, while wood is readily available in considerable quantity, bone material is an extremely limited resource that must be used in an extremely efficient manner.
Various types of bone grafts are known. For example, as disclosed in U.S. Pat. No. 5,989,289 to Coates et al., a spinal spacer includes a body formed of a bone composition such as cortical bone. The spacer has walls that define a chamber that is sized to receive an osteogenic composition to facilitate bone growth.
U.S. Pat. No. 5,899,939 to Boyce et al. discloses a bone-derived implant for load-supporting applications. The implant has one or more layers of fully mineralized or partially demineralized cortical bone and, optionally, one or more layers of some other material. The layers constituting the implant are assembled into a unitary structure, as by joining layers to each other in edge-to-edge fashion in a manner analogous to planking.
Another bone-grafting material is disclosed in U.S. Pat. No. 4,678,470 to Nashef et al., and is formed using a tanning procedure involving glutaraldehyde that renders the material osteoinvasive. A bone block is shaped into a precise predetermined form and size using conventional machining techniques. A paste-like suspension is also formed using known methods of comminuting bone, such as milling, grinding, and pulverizing, and adding the pulverized or powdered bone to a carrier. The treatment with glutaraldehyde allows the use of bovine, ovine, equine, and porcine bone sources. However, if the final desired form of the bone grafting material is a block of bone or machined shape, the bone stock must be large enough to provide a block of the required size.
U.S. Pat. No. 5,981,828 to Nelson et al. discloses a “composite” acetabular allograft cup for use in hip replacement surgery. A press is used to form the cup from impacted cancellous bone chips and cement. The composite is a hollow hemispherical dome having an outer surface comprised essentially of exposed cancellous bone chips and an inner surface comprised essentially of hardened bone cement. The cancellous bone chips are first placed in a mold and subjected to a load to form a compact and consolidated mass that conforms to the shape of the mold. The mold is then opened, cement is applied, and the mold is then reapplied. While an allograft of a particular shape may be formed using this process, the process is limited to forming an allograft by compressing cancellous bone chips. Thus, numerous molds are required in order to produce allografts of different sizes, and the use of bulk-size allograft source material is not facilitated.
With a rapidly increasing demand in the medical profession for devices incorporating bone material, the tremendous need for the tissue material itself, particularly allograft tissue material, presents a considerable challenge to the industry that supplies the material. Due to the size and shape of the bones from which the material is harvested, and the dimensional limitations of any particular type of bone in terms of naturally occurring length and thickness (i.e. cortical or cancellous), there is a need for a means by which individual bone fragments can be combined to form larger, integral implants that are more suitable for use in areas of larger fractures or defects. For example, the size of cortical bone fragments needed to repair a fracture or defect site is often not available in a thick enough form. While multiple fragments may together meet the size and shape requirements, several prominent concerns have placed a practical limitation on the implementation of this concept. There is considerable uncertainty regarding the structural integrity provided by fragments positioned adjacent to one another without bonding or other means of securing the fragments to each other. Moreover, there is concern over the possibility that a fragment may slip out of position, resulting in migration of the fragment and possible further damage in or near the area of implantation.
In addition, due to the geometry of bones such as the femur and tibia, all portions of the bones are not readily usable as a result of size limitations. Thus, prior art implants, specifically allografts, are produced with an inefficient use of source bones.
Turning to exemplar uses for implants formed from bone, a number of medical conditions such as compression of spinal cord nerve roots, degenerative disc disease, and spondylolisthesis can cause severe low back pain. Intervertebral fusion is a surgical method of alleviating low back pain. In posterior lumbar interbody fusion (“PLIF”), two adjacent vertebral bodies are fused together by removing the affected disc and inserting an implant that would allow for bone to grow between the two vertebral bodies to bridge the gap left by the disc removal.
A number of different implants and implant materials have been used in PLIF with varying success. Current implants used for PLIF include threaded titanium cages and allografts. Threaded titanium cages suffer from the disadvantage of requiring drilling and tapping of the vertebral end plates for insertion. In addition, the incidence of subsidence in long term use is not known. Due to MRI incompatibility of titanium, determining fusion is problematic. Finally, restoration of lordosis, i.e., the natural curvature of the lumbar spine is very difficult when a cylindrical titanium cage is used.
As discussed above, allografts are sections of bone that may be taken from a long bone of a donor. A cross section of the bone is taken and processed using known techniques to preserve the allograft until implantation and reduce the risk of an adverse immunological response when implanted. For example, U.S. Pat. No. 4,678,470 discloses a method for processing a bone grafting material which uses glutaraldehyde tanning to produce a non-antigenic, biocompatible material. Allografts have mechanical properties which are similar to the mechanical properties of vertebrae even after processing. This prevents stress shielding that occurs with metallic implants. They are also MRI compatible so that fusion can be more accurately ascertained and promote the formation of bone, i.e., osteoconductive. Although the osteoconductive nature of the allograft provides a biological interlocking between the allograft and the vertebrae for long term mechanical strength, initial and short term mechanical strength of the interface between the allograft and the vertebrae are lacking as evidenced by the possibility of the allograft being expelled after implantation.
Currently commercially available allografts are simply sections of bone not specifically designed for use in PLIF. As a result, the fusion of the vertebral bodies does not occur in optimal anatomical position. A surgeon may do some minimal intraoperative shaping and sizing to customize the allograft for the patient's spinal anatomy. However, significant shaping and sizing of the allograft is not possible due to the nature of the allograft. Even if extensive shaping and sizing were possible, a surgeon's ability to manually shape and size the allograft to the desired dimensions is severely limited.
Most PLIF implants, whether threaded cages or allograft, are available in different sizes and have widths that vary with the implant height. For example, the width of a cylindrical cages will be substantially equivalent to the height. Although larger heights may be clinically indicated, wider implants are generally not desirable since increased width requires removal of more of the facet, which can lead to decreases stability, and more retraction of nerve roots, which can lead to temporary or permanent nerve damage.
There is a need for new, fundamental approaches to working with and processing tissues, in particular allograft material, especially with regard to machining, mating, and assembling bone fragments. Specifically, there is a need for an implant that allows more efficient use of source material. More specifically, there is a need for an implant that is an integrated implant comprising two or more bone fragments that are interlocked to form a mechanically effective, strong unit. There also is a need for an improved implant for fusing vertebrae.